In illuminated environments, changed ambient conditions can change the illumination conditions or brightnesses. By way of example, the ambient conditions can change with regard to the current illumination, which corresponds to short-term processes, and as a result of ageing processes in the luminous sources used, which corresponds to long-term processes. Changed ambient conditions with regard to the illumination arise, for example, in the case of changed light incidence through windows into an illuminated room at different times of day. Surface light sources such as an organic light emitting diode (OLED), for instance, can furthermore be subject to ageing processes, as a result of which the luminance in total decreases over time depending on the OLED layer construction and processing. The decrease in the luminance can be caused, for example, by elevated temperatures which can occur during operation and can damage the organic materials.
In order to keep the luminance in the environment of a luminous source such as a surface light source, for instance, constant over time, the light emitted by the luminous source can be regulated, for example, by means of manual dimming or by means of an electronic circuit which controls the luminous source on the basis of a measurement signal from one or a plurality of externally connected sensors. As external sensors it is possible to use, for example, photodiodes, photoconductors, phototransistors or photothyristors for detecting the entire radiation power emitted by the luminous source at a preselected location, and, by means of an external interconnection or wiring, they can be part of a control of the luminous source. However, such a possibility usually requires a high complexity and causes additional costs. Manual dimming, by contrast, enables only imprecise coordination with lighting conditions actually present and causes an unnecessary waste of energy and, under certain circumstances, incorrect illumination conditions.
Since the known control possibilities for surface light sources have a high interconnection complexity or cannot be automated, an automatic readjustment of the luminance is not possible without considerable additional outlay.